Microscopes comprise at least two imaging optical elements, namely a main objective and an eyepiece. All optical observation devices possess, in principle, the same fundamental construction. The two optical elements (objective and eyepiece) form an overall system that defines the potential magnification range as well as the depth of field. In most optical devices the depth of field, i.e. the region in which one or more specimen points located on the optical axis are imaged in focus at the same time, is very limited. An increase in depth of field is desirable in order to present a more comprehensive image to the observer. In many microscope application areas, in particular those for surgical microscopes (e.g. for neurosurgery), a particularly high degree of depth of field is important because the surgeon must view, sharply, not only the plane of focus but also as many specimen regions as possible in front of and behind the plane of focus.
In order to increase depth of field, it is known to increase the focal length of the lens system of the optical device and/or to make the aperture smaller. In the field of application of microscopes, the former possibility is almost excluded since the focal length range is limited. Because the depth of field decreases with increasing microscope magnification, only a thin object plane can be imaged sharply. The second possibility for increasing depth of field, by making the aperture smaller, results in a decrease in image brightness and a loss of resolution and contrast.
An optical device according to the present invention for imaging a specimen is equipped with a main objective and a device for modifying depth of field, the device for modifying depth of field comprising a micromirror array having micromirrors that are individually controllable and adjustable as to their spatial orientation.
Micromirror arrays as such are known from the existing art. They are generally made up of a two-dimensional arrangement of individual mirrors whose positions are adjustable individually or in suitable combination. The micromirrors are joined to a stationary support element. By appropriate adjustment of the micromirrors while the support element remains stationary, it is possible to implement a beam deflection system that consequently requires little space. The absence of a need for pivoting of a support element or deflection element thus contributes to a small overall height for the optical device (microscope), and permits highly accurate alignment and almost vibration-free operation.
Because the micromirrors can assume various angular positions relative to one another, two or more different deflection angles can be implemented simultaneously with one micromirror array. The adjustability of the micromirrors makes possible, in particular, a flexible selection of the deflection angle. For example, the spatial orientation of the micromirrors (and therefore the deflection angle) can be modified as a function of time. On the other hand, it is possible to implement (statically) two or more deflection angles by corresponding orientation of the micromirrors on the micromirror array, so that beam paths arriving from one direction are diverted in different directions or, as utilized for the present invention, beam paths arriving from different directions are diverted in the same direction.
It is thus possible in this fashion for specimen points that are located outside the focal plane and can no longer be imaged (or can no longer be imaged sharply) by the optical device to be imaged by the optical device in a context of appropriate adjustment of the micromirrors.
In an advantageous embodiment of the invention, at least a portion of the micromirrors of the micromirror array are consequently adjustable in such a way that an observation ray bundle or beam path of a specimen point located outside the focal plane is imaged, as a result of a substantially spherical spatial orientation of the relevant micromirrors, in the same fashion as a specimen point located in the focal plane in the context of micromirrors oriented substantially parallel to one another.
It is additionally advantageous if the micromirrors of the micromirror array are adjustable in such a way that multiple specimen points located along the optical axis proceeding through the main objective are each imaged in the same fashion, simultaneously or in chronological sequence, by means of differing static orientations of the micromirrors or by means of an orientation of the micromirrors that changes over time. As already discussed, different deflection angles (and therefore orientations) of the relevant micromirrors can be implemented in a micromirror array both simultaneously and in chronological sequence. By suitable orientation of the relevant micromirrors, two or more specimen points located in the direction of the optical axis can thus simultaneously be effectively imaged into infinity by means of the main objective and the micromirror array, and thus sharply imaged by the optical device, with the result that depth of field is increased.
On the other hand, in the context of a chronologically sequential change in the orientation of the relevant micromirrors, an entire range of specimen points located along the optical axis can also be effectively imaged into infinity by means of the main objective and the micromirror array, with the result that depth of field is increased in the same fashion.
In the former case of simultaneous imaging of different specimen points on the optical axis, it is useful to place these specimen points so that the depth-of-field regions associated with them (present in system-inherent fashion as manufactured) end up located adjacently to one another. A large depth of field can thus be attained by summation of depth-of-field ranges.
In the case of chronologically sequential imaging of a range of specimen points located on the optical axis, it is useful to run through this range, by corresponding modifications of the micromirror orientations, so quickly that the observer (human eye or camera) has the impression of a static image. This is the case when the runthrough frequency is greater than or equal to the respective so-called flicker fusion frequency.
It must be noted, in the context of the optical device according to the present invention, that the end result of the device for modifying the depth of field is to modify the effective focal length of the main objective. This resulting variation in the effective focal length of the main objective (the invention thus implements a variable objective) causes, in the context of a microscope, a change in the microscope magnification and therefore a pulsing of the image and a distortion of the stereoscopic impression. The range of specimen points to be imaged should consequently be selected correspondingly so that these negative effects are not too greatly evident. Furthermore, in an advantageous embodiment, an attempt can be made to counteract these effects by largely compensating for the resulting changes in microscope magnification by way of an opposite-direction change in the zoom magnification of a downstream zoom system. Rapid application of control to the zoom system, with short reaction times, is necessary for this purpose.
As has already been noted repeatedly, it is advantageous if the optical device is a microscope, in particular a stereomicroscope, in particular a surgical microscope.
The device according to the present invention, configured as a micromirror array, for modifying depth of field also allows implementation of a variable objective, as already discussed. By way of a corresponding spherical orientation of the micromirrors of the micromirror array, the focal length of the main objective can be effectively modified with no need to effect changes to the main objective itself.
The rapid, delay-free, low-vibration adjustability of the micromirrors of a micromirror array makes possible in user-friendly fashion, especially in the context of high-precision devices such as surgical microscopes, specimen examination with increased depth of field or with a modifiable main objective focal length.